I should say right away that ALL of my decks are about 72 cards, despite the fact that all tournament decks have exactly 60 cards.
Why? Because I enjoy the variety that a larger deck allows, particularly when played repeatedly against the same opponents.
Besides, tournament decks have 15-card sideboards to provide some variety, and since I seldom have the opportunity to employ a sideboard in a friendly game,
I just shuffle in some extra cards and add some extra lands. Purists can bemoan this all they like, but it's more fun.

Firestorm (Red/White): This is my most powerful deck. Very direct and to the point: Fry everything, including your opponent.
White is used solely to get rid of COP:Red and other pesky enchantments.

Desecration (Red/Black/Blue): This is one of those decks that's just annoying to play against.
It's mana-denial, with some big creatures to finish your opponent.
The deck works well in dual play, but don't try it in a group game -
you'll never get rid of enough of your opponents' land to do more than piss them off,
and your defenses will be fairly anemic, so they'll make a point of crushing you early. (BTDT)

Goblin Hordes (Red): What else can you do with all those Goblins?
Coat of Arms makes this a surprisingly effective deck...

Big Black Shadow (Black/White): Shadow is the ultimate anti-blocking ability, since it's new and few creatures have it.
Shadow creatures can only be blocked by other shadow creatures, but there's a catch: they can't block normal creatures.
Thus, a deck with all shadow creatures would fall prey to normal creature decks,
so this deck uses a mix of shadow creatures and big black creatures to overrun your opponent.
White is used to get rid of enchantments and the ever-present White Knights.

Altar of Sacrifice (Black/Green): This deck was inspired by all the powerful Black cards (like Pit Fiend) that require creature sacrifice.
Why not pair these cards with Green and Black creature-generators, and actually use them?
Of course, a Verdant Force can do plenty of damage on its own while it continues to churn out Saprolings...

Tim's Family (Blue/Red): This was something of an experiment: creating a direct-damage deck that's NOT red.
It's loaded with Prodigal Sorcerers and Mawcors, but it's not one of my most powerful decks.

Hand Denial (Black/Blue): I don't think there's anything more irritating to face than a hand-denial deck.
I got so sick of playing against my cousin's that I made this one to give him a taste of his own medicine.

Poison (Black/Green): Every time I bring this deck into play, my opponents laugh at me. And every time, I win.
Though this says something about the advantage of being underestimated, it also says something about the effectiveness of poison in a well-crafted deck.

Wurms & Dragons (Green/Red): I designed this deck for use in multi-player games, when big creatures are most needed.
These cards are fun to play, and the deck can fare quite well in dual play if you can get your opponent on the defensive.
"Gosh, do I attack, or save something to block his Shivan Dragon?"

Nature's Wrath (Green): This is the ultimate big green creature deck.
No two creatures are the same, so it makes for a different game every time.

Live and Let Die (Green/White): There's an obscure rule in Magic that a player who reaches 100 life wins the game -
no matter how many players are involved.
This deck is designed to get you to 100 life QUICKLY, particularly in a multi-player game.
Unless your opponents catch on early and attack you en masse, this deck will garner a surprise win and some major bragging rights.

Emmissaries of God (White): This is a big white creature deck composed mainly of angels.
Makes for a fun game, particularly if you're the type of person who played a Paladin in D&D.

The Meek (White): Though also white, this deck is the antithesis of the Emmissaries of God.
It relies on a horde of small creatures to overrun the opponent. Banding is used to enhance those creatures.

Islandhome/Islandwalk (Blue): Blue has the biggest creatures in the game (Polar Kraken, Leviathan, Benthic Behemoth, etc.),
and it's easy to transform some of your opponents' lands to islands, then walk these creatures over for the kill.

The Uncastables (Gold): This is a five-color deck that's built around Slivers and the Gold Dragons.
I honestly haven't played it enough yet to comment on its effectiveness, though it's certainly a lot of fun!